Connection Stiffness

Connection Stiffness allows for better approximation of axial and shear stiffness in grounded fasteners, grounded joints, and cylindrical supports.

Fastener flexibility and joint flexibility in general can have a significant effect on how loads are carried in and transferred through a structure.

Numerous methods exist to estimate fastener stiffness. Inspire has implemented the Huth-Schwarmann method, but also allows you to specify an independently calculated stiffness.



In an idealized infinitely stiff connection as shown below, the amount of load transferred to the center rivet is 0%, with 50% on each of the outer rivets.


If the connections are infinitely soft, then the load will be transferred equally between rivets.


For real fasteners, the actual load carried will be somewhere between these limit values. For example, 30% on the center rivet and 35% on each of the outer rivets.
For connections to and between plates, as are often found in the aircraft industry, a popular way to approximate the fastener stiffness is by using the Huth-Schwarmann method. This stiffness is calculated using the following formula:


Where n = 1 for single shear and n = 2 for double shear, and a and b depend on the type of joint as per the following table:
Type a b
Bolted metallic 2/3 3.0
Riveted metallic 2/5 2.2
Bolted graphite/epoxy 2/3 4.2
The following parameters also apply:
Configuration:
d = hole diameter
t = plate thickness
Material:
E = Young's modulus
ν = Poisson's ratio
Indices:
1 = Plate 1 (central one in double shear)
2 = Plate 2 (outer ones in double shear)
f = fastener

As an alternative to using the Huth-Schwarmann stiffness, user-defined stiffness values can also be applied. This is a more common approach when the parts can't be defined using a plate approximation. One example would be a cast aluminum automotive mount attached to a transmission housing. Rather than model the entire system, if only the mount is of interest, the connection stiffness can be approximated and used to give improved results.

Connection Stiffness Properties

Following is a list of connection stiffness properties and the types of objects to which they apply (indicated in parentheses).

Pressing F3 will open the Property Editor.

Type (grounded fasteners, grounded joints, cylindrical supports)
Select what type of connection stiffness to use for grounded fasteners, grounded joints, or cylindrical supports. The default is Rigid, or you can select Huth-Schwarmann or User Defined.
  • Default: Both the axial stiffness and shear stiffness are set to Rigid.
  • Huth-Schwarmann: The axial stiffness is set to Rigid, and the shear stiffness is calculated using the modulus of elasticity of the material, the hole size, and an assumption that the bolt is ground to a part of the same thickness and material (single shear joint).
  • User Defined: Both the axial stiffness and shear stiffness can be manually entered by the user.
Axial Stiffness (grounded fasteners, grounded joints, cylindrical supports)
The spring stiffness in the axial direction.
Shear Stiffness (grounded fasteners, grounded joints, cylindrical supports)
The spring stiffness in the shearing direction.